Implantable cardiac devices are well known in the art. They may take the form of implantable defibrillators or cardioverters which treat accelerated rhythms of the heart such as fibrillation or implantable pacemakers which maintain the heart rate above a prescribed limit, such as, for example, to treat a bradycardia. Implantable cardiac devices are also known which incorporate both a pacemaker and a defibrillator.
A pacemaker may be considered as a pacing system. The pacing system is comprised of two major components. One component is a pulse generator which generates the pacing stimulation pulses and includes the electronic circuitry and the power cell or battery. The other component is the lead, or leads, having electrodes which electrically couple the pacemaker to the heart. A lead may provide both unipolar and bipolar pacing polarity electrode configurations. In unipolar pacing, the pacing stimulation pulses are applied between a single electrode carried by the load, in electrical contact with the desired heart chamber, and the pulse generator case. The electrode serves as the cathode (negative pole) and the case serves as the anode (positive pole). In bipolar pacing, the pacing stimulation pulses are applied between a pair of closely spaced electrodes carried by the lead, in electrical contact with the desired heart chamber, one electrode serving as the anode and the other electrode serving as the cathode.
Pacemakers deliver pacing pulses to the heart to cause the stimulated heart chamber to contract when the patient's own intrinsic rhythm fails. To this end, pacemakers include sensing circuits that sense cardiac activity for the detection of intrinsic cardiac events such as intrinsic atrial events (P waves) and intrinsic ventricular events (R waves). By monitoring such P waves and/or R waves, the pacemaker circuits are able to determine the intrinsic rhythm of the heart and provide stimulation pacing pulses that force atrial and/or ventricular depolarizations at appropriate times in the cardiac cycle when required to help stabilize the electrical rhythm of the heart.
Pacemakers are described as single-chamber or dual-chamber systems. A single-chamber system stimulates and senses the same chamber of the heart (atrium or ventricle). A dual-chamber system stimulates and/or senses in both chambers of the heart (atrium and ventricle). Dual-chamber systems may typically be programmed to operate in either a dual-chamber mode or a single-chamber mode.
The energies of the applied pacing pulses are selected to be above the pacing energy stimulation threshold of the respective heart chamber to cause the heart muscle of that chamber to depolarize or contract. If an applied pacing pulse has an energy below the pacing energy stimulation threshold of the respective chamber, the pacing pulse will be ineffective in causing the heart muscle of the respective chamber to depolarize or contract. As a result, there will be failure in sustaining the pumping action of the heart. It is therefore necessary to utilize applied pacing pulse energies which are assured of being above the pacing energy stimulation threshold.
However, it is also desirable to employ pacing energies which are not exorbitantly above the stimulation threshold. The reason for this is that pacemakers are implanted devices and rely solely on battery power. Using pacing energies that are too much above the stimulation threshold would result in early depletion of the battery and hence premature device replacement. Prior to autocapture, the capture threshold would be assessed at the periodic follow-up visits with the physician and the output of the pacemaker adjusted (programmed) to a safety margin that was appropriate based on the results of that evaluation. However, capture thresholds may change between scheduled follow-up visits with the physician. A refinement of the technique of periodic capture threshold measurements by the physician was the automatic performance of capture threshold assessment and the automatic adjustment of the output of the pulse generator. Capture threshold may be defined in terms of pulse amplitude, either voltage or current, pulse duration or width, pulse energy, pulse charge or current density. The parameters that can be easily adjusted by the clinician are pulse amplitude and pulse width. With the introduction of autocapture, the implanted pacing system periodically and automatically assesses the capture threshold and then adjusts the delivered output. It also monitors capture on a beat-by-beat basis such that a rise in capture threshold will be recognized allowing the system to compensate by delivery initially of higher-output back-up or safety pulses and then incrementing the output of the primary pulse until stable capture is again demonstrated. The output amplitude of the pacing stimulus is set slightly above the measured capture threshold minimizing battery drain while the patient is protected by the significantly higher output back-up safety pulse. These evaluations are often referred to as autocapture tests or simply autocapture.
As is well known in the art, the stimulation threshold of a heart chamber can, for various reasons, change over time. Hence, pacemakers that incorporate autocapture are generally able to periodically and automatically perform autocapture tests. In this way, the variations or changes in stimulation threshold can be accommodated.
When a pacing pulse is effective in causing depolarization or contraction of the heart muscle, it is referred to as “capture” of the heart. Conversely, when a pacing pulse is ineffective in causing depolarization or contraction of the heart muscle, it is referred to as “lack of capture” or “loss of capture” of the heart.
When a pacemaker performs an autocapture test, its pulse generator applies a succession of primary pacing pulses to the heart at a basic rate. The output of the primary pulse is progressively reduced. In one known system, there will be a minimum of two consecutive pulses at a given energy before the output associated with the primary pulse is reduced or increased. The output of successive primary pacing pulses is reduced by a known amount and capture is verified following each pulse. If a primary pulse results in loss of capture, a backup or safety pulse is applied to sustain heart activity. If there is loss of capture associated with the primary pulse on two successive cycles, this is interpreted as being subthreshold. At that time, the output is progressively increased in small increments until capture is confirmed on two consecutive primary pulses. This, of course, is but one example. As is known in the art, a single pulse or any number of pulses may be used. The lowest output setting that results in capture on consecutive pulses starting from a low value where there is loss of capture is defined as the capture threshold. The present system then automatically adjusts the output with a working margin of an additional 0.25 Volts. In these methods, capture may be verified by detecting the evoked response associated with the output pulse, the T-waves, mechanical heart contraction, changes in cardiac blood volume impedance, or another signature of a contracting chamber.
Loss of capture can have many different causes. A common causes involves lead failure. Lead failure may result, for example, when the two conductors of a bipolar pacing lead become shorted together. Another lead failure may involve an open circuit where the continuity of one or both conductors in a bipolar lead is disrupted. In the event of either occurrence, switching from a bipolar pacing polarity electrode configuration to a unipolar pacing polarity electrode configuration may restore stimulation effectiveness.
Pacemakers are also capable of sensing. When programmed to the bipolar sensing configuration, the signal that is detected is the voltage difference between the two active electrodes inside the heart. In a unipolar sensing configuration, the signal that is detected is the voltage difference between one electrode in the heart and an electrode located elsewhere. Most commonly, this is the metallic housing of the pulse generator. Unipolar sensing can also be further specified as being between the electrode tip inside the heart and the housing of the pulse generator or between the proximal ring electrode that is set back from the tip and the housing of the pulse generator.
In one implementation of autocapture, the output configuration may be unipolar and the sensing configuration may be bipolar. Further, in the current implementation, autocapture may not be enabled with a dedicated unipolar lead or a bipolar lead programmed to a unipolar sensing configuration.
While autocapture is generally used with ventricular leads, it may be used with atrial leads as well. Hence, it is to be understood that the invention is applicable for either the atrial and/or ventricular leads.
Pacemakers are known having lead supervision wherein lead impedance is measured on either a beat-to-beat or more commonly, periodic basis. If the lead impedance is above or below a certain threshold, the pacing electrode configuration may be automatically switched to a unipolar pacing electrode configuration.
Impedance measurement on a beat-to-beat basis increases the power consumption of the implanted device and consequently reduces the longevity of the device. Assessing lead impedance, also called stimulation impedance, has the same limitations as assessing the capture threshold on a periodic but infrequent basis. Problems may be manifest between scheduled evaluations leaving the patient unprotected if a problem were to develop. Still further, if autocapture were to be enabled, the output configuration is unipolar and impedance measurements would be made in the unipolar configuration. A mechanical problem developing in a lead is likely to be manifest in the bipolar configuration first. As such, an early manifestation of a lead malfunction may not be appreciated in the unipolar output configuration. If a problem were detected, reverting to the unipolar sensing configuration based upon lead impedance may require that autocapture be disabled. The present invention addresses these issues by providing an implantable cardiac stimulation system which conserves energy while providing lead supervision when required, maintaining autocapture, and most importantly, continued stimulation of the heart in the event of a lead failure.